A typical down-the-hole hammer comprises an external cylinder or outer wear sleeve, within which is mounted an inner cylinder, this in turn engaging with a backhead assembly. The backhead assembly is connected to source of compressed fluid via a drill rod. A sliding reciprocating piston co-operates with the inner cylinder and backhead assembly such that when compressed air is supplied through the backhead assembly, the piston acts with percussive effect on a drill bit retained within a chuck on the outer wear sleeve.
In some drilling applications, the drilled hole may cave in, and debris from the walls of the hole may fall down on the hammer. Such debris often lodges at the connection between the backhead and the drill rod, because the drill rod diameter is typically smaller than the diameter of the hammer and causes a restriction which impedes withdrawal of the hammer from the drilled hole. When this happens, the hammer is pulled back against the restriction. The hammer must be pulled with sufficient force to pull it past the restriction. However, where the restriction is closely packed, it may not be possible to pull the hammer through the restriction and in such cases, the hammer may be lost in the drilled hole.
In prior art systems, a back hammer, such as that produced by PG Drilling Equipment USA LLC, may be installed in the drill string above the drilled hole when required. There are a number of disadvantages associated with such a system. The first is that the back hammer is attached in the drill string at the joint nearest to the restriction. The back hammer is therefore positioned above the restriction and simply shakes the drill rod in an attempt to free it from the restriction. A further disadvantage is that the system is always on, that is, once installed, the back hammer operates regardless of whether a restriction is encountered or not.
Another prior art system is described in U.S. Pat. No. 5,119,891 which provides an adaptor for drilling strings, the adaptor having a controllable air passage. The adaptor comprises a body mountable on the lower end of a drill string and a sleeve rotatably mounted on the body and longitudinally moveable relative thereto. In normal drilling operation, pressurized air from a central pressure line of the drill string is directed via a central bore in the body to a first chamber defined by the body and the sleeve and through to a second chamber defined by the body and the sleeve and to the air intake of the hammer or drill bit. When the adaptor is to be used to clear a restriction above the hammer, the rotation of the drill string is stopped and the sleeve moves down relative to the body, such that the pressurized air can exhaust through exhaust passages above the sleeve to clear the debris. The sleeve may also be provided with teeth at an upper end thereof and the drill string may be rotated to enable the teeth to cut through the obstruction in the drill hole.
The primary disadvantage of this system is that, once the system is opened to allow air to exhaust through the restriction, it cannot be closed unless the hammer is placed back on the hole bottom or there is sufficient friction with the hole wall to encourage the system to close when rotating in the correct direction. Air thus continues to flush above the sleeve even after the restriction has been cleared. A further disadvantage is that the primary mechanism for removal of the restriction is the air flushed from the hammer. In certain cases this may not be sufficient to clear debris from a drilled hole to allow the hammer to be withdrawn. Although cutting teeth are also provided, again, these may not be sufficient to clear the restriction. Yet another disadvantage is that the helical splines are difficult to manufacture.
It is therefore desirable to provide a system which allows a percussion drill tool to be withdrawn from a drilled hole where a restriction has occurred while avoiding many of the disadvantages of prior art systems. In particular, it is desirable to provide a system for back hammering through a restriction which is operable only when a restriction is encountered. It is also desirable to provide a system which closes to prevent flushing once the restriction is cleared.